1. Field of the Invention
The present invention relates to a display using a light-emitting element. In particular, the invention relates to a display that has improved gradation reproduction and is capable of multi-gradation reproduction.
2. Description of the Related Art
Recently, flat panel displays with a thin shape and light weight have been used in wide fields in replace of Braun tubes (CRTs), and applications thereof have been expanded. This has resulted from the accelerated spread of personal information terminals such as personal computers and cellular telephones compatible with network access, due to the development of information devices and infrastructure for service networks using the Internet as a core. In addition, the market for flat panel displays has expanded to home use television sets, which was conventionally the exclusive market of CRTs.
Liquid crystal displays (LCDs) have already come to occupy a large market share due to such characteristics as a thin shape and light weight and low power consumption, but there are disadvantages to be improved with respect to some display characteristics, such as a viewing angle, contrast and response speed. Accordingly, on the one hand, the improvement of LCDs themselves has been advanced, and research and development with respect to displays based on completely different devices or principles have also been actively conducted.
Among these, as a device recently getting a lot of attention in particular, there is an organic electroluminescence display (OELD). An OELD is a display that emits light corresponding to electric signals and is constituted using an organic compound as a light-emitting material. The OELD inherently has excellent display characteristics such as a wide viewing angle, high contrast and high-speed response. Further, there is a possibility that it can realize displays of from a small size to a large size with a thin shape and light weight and a high image quality. Therefore, it has attracted attention as a display capable of replacing CRTs and LCDs.
Concerning a driving technique of a matrix panel arranged with plural pixels, for OELDs as well, both duty driving (time-division driving) and static driving have been developed, similarly as in the case of displays such as LCDs.
The former has a simple panel structure resulting in a simple and low cost process. However, when the number of scanning lines increases, the emission period of respective lines relative to the period for scanning the whole screen decreases. As a result, for the purpose of obtaining required panel brightness, the peak brightness of pixels becomes high. When compared with driving conditions of full-time lighting, usually, this results in a poor light-emission efficiency to require driving by high voltage or current.
Further, due to power loss in wiring portions, the larger a screen becomes, the more disadvantageous this is with respect to power consumption.
In contrast, since the latter is combined with nonlinear elements such as a thin film transistor (TFT), the process becomes complex. However, since such a constitution is possible in which an emission duration is maintained for the scanning period of one line, or longer, low power consumption and long operating life can be expected due to the decrease in the peak brightness of pixels and current.
As the TFT, a TFT of poly-silicon (p-Si) type such as continuous grain silicon (CGS) that has a high density patterning and high current driving performance is preferable. A high mobility and integration property, which are characteristics of the p-Si type TFT as an element, also make it possible to build such constituents as a driving IC (integrated circuit) and control circuit into a panel. For these reasons, technical development for active driving of an OELD is currently the main stream.
When performing active driving, the driving current necessary for obtaining a predetermined brightness per one pixel is several micro amperes at the maximum, and, in order to obtain a wide range of gradation reproduction, the minimum difference in current between respective gradations is a very minute value of several tens of nanoamperes. Control of variation of a TFT manufactured in an ordinary manner occurs at a range of current value that exceeds this minute current value. Therefore, there occurs such gradation trouble as the reversion of brightness due to the variation. Accordingly, as a method for controlling such a very minute current with a TFT, for example, Japanese Patent Application Laid-Open (JP-A) No. 2001-147659 discloses generating a driving current with an exterior driving circuit, performing writing to a writing capacitor in a pixel by a driving current generated through the improvement of a TFT circuit in a pixel to enable an organic EL element to be driven with the driving current, thereby compensating for the variation in characteristics of a TFT in the pixel circuit to reduce the brightness variation. However, there are such problems to be solved in that one more TFT is required for controlling the TFT and heavy equipment is required for lowering the current and voltage of a TFT driving device.
Along with even more fine reproduction of a display, and further improvement of light-emission efficiency of a light-emitting element, the minimum current value to be controlled tends to be reduced more and more. Thus, a controlling method that has higher reliability and is convenient and compact is required.
JP-A No. 2003-280593, for example, discloses a display provided with a first sub-pixel for displaying multi-gradation including a halftone and a second sub-pixel having a lower number of gradations. The first sub-pixel is controlled by an offset voltage switching means that switches an offset voltage on the basis of the comparison between an analog signal and a reference signal, and the second sub-pixel is controlled by binary values of lightness and darkness. However, organic electroluminescence elements by voltage control have such a problem in that the brightness varies largely according to temperature variation, and are not practical. On the other hand, JP-A No. 5-34702 discloses a liquid crystal display provided with sub-pixels having different areas from each other. The gradation of the liquid crystal display is also controlled by voltage, but, for a display in which gradation is controlled by current, it is impossible to perform gradation reproduction with a high reliability only by providing sub-pixels having different areas from each other.
Regarding the current control and voltage control in an organic EL element, there is detailed description in “Organic EL Materials and Displays (Yuuki EL Zairyou to Disupurei)” edited by Junji Kido, pages 283-284, CMC (2001). Driving of an organic EL element is based on direct current driving in which a hole and an electron are injected into a light-emitting layer by applying a direct current electric field between an anode and a cathode, and the driving results from the fact that the emission brightness in the organic EL is proportional to a value of the driving current. As shown on page 284, FIG. 3 of the above reference, the current and the brightness show very good linear relationship, and therefore, it is understood that the brightness can be stably controlled by current values. However, as shown in FIG. 4 on the same page of the reference, voltage and brightness are in a relationship similar to ON/OFF characteristics, which is suitable for pulse modulation control, but, in the case where it is used for the brightness control of an organic EL element, the brightness varies significantly according to a slight variation in voltage, which is not preferable. In particular, it is extremely difficult to display an image having multi-gradation with good reproduction by a voltage control system.
Even in the case of a current control system, multi-gradation reproduction of images is not yet sufficient, and means for image reproduction having multi-gradation by a current control system is desired even further.